The decoration of ceramic surfaces by the so-called silk screen process is well established. A screen of fine metal wires is used in which selected areas of the screen are blocked or masked so as to pass through the screen a decorating ink or paint in a predetermined pattern only, the pattern defining the print to be deposited on the ceramic. This practice has encountered a number of problems arising in particular with the demands of ever increasing speed in screen application, multi-color screen printing, and economic considerations in screen-printing disposable or throwaway containers.
For many years screen printing of glass such as glass enamel, has been carried out by dispersing a pigment and ground vitreous frit in a liquid vehicle, such as a viscous oil, squeegeeing the dispersion through a masked screen onto the glass surface, and then firing the glass to drive off the organic constituents of the dispersion and fix the pigment to the glass surface. When the speed requirements of screen printing are not great and/or highly durable, fired prints are desired, this process is quite satisfactory. There are disadvantages with this process, however, when multi-color, superimposed or adjoining prints are desired. In these cases, each screening operation must be followed by air-drying and hardening steps, often involving oven equipment, before a succeeding screen application can be made on the same glass ware, followed in turn by still additional drying prior to screen application of another color, etc., in order to prevent smearing and blending of one color into another before the ware is ultimately fired at a relatively high temperature, for example, 1200.degree. F.
To eliminate the necessity for a drying operation after each screen printing, particularly in multi-color applications, hot melt or thermo-fluid ceramic inks have been developed. These inks typically comprise a thermoplastic vehicle and a glass binder. They are solid at the temperature of the ware to be printed (normally room temperature) and become fluid and therefore flowable when heated to a higher temperature. In the fluid condition, thermo-fluid ceramic inks are forced through a screen in the usual manner onto a substrate to be printed where they solidify almost instantly because of the relatively low temperature of that substrate. These inks may be maintained in a fluid condition either by radiant heat or by passing an electric current through the wires of the screen.
In order to function properly, such thermo-fluid ceramic inks must have certain physical properties. Thermo-fluid ceramic inks must melt quickly within a narrow temperature range without substantial physical or chemical change; they must not run after application to a surface; and they must level properly before solidifying. If the thermo-fluid ceramic ink has insufficient fluidity, tiny pinholes result in the design left by the screening operation. The pinholes later enlarge during a firing operation and permanently detract from the appearance of the printed design. Pinholes also tend to promote bleeding of subsequently screened colors into other colors during the time required for the subsequently screened colors to solidify.
Other requirements for a satisfactory thermo-fluid ceramic ink include good adherence to ceramic, metal or glass surfaces; avoidance of gelation when in a molten condition; stability over prolonged periods of time; moisture-resistance; ability to be fired without leaving objectionable carbon deposits; and, where required, the ability to meet various hot alkali tests.
In view of these essential and sometimes diverse specifications, difficulty has been met in producing thermo-fluid ceramic inks that meet all these requirements and that are uniformly suitable for various screen applications. Previously, the art has found it necessary to compound thermo-fluid inks from a fairly large number of components in order that the inks have as many as possible of the listed desired physical properties. One or more natural waxes in combination with one or more natural resins in combination with still other ingredients have been suggested. For example, U.S. Pat. No. 2,748,093 discloses a vehicle suitable for the application of enamel by a screen process, the vehicle comprising diphenyl, hydrogenated rosin, an ethylene glycol ester of hydrogenated rosin, a diethylene glycol ester of hydrogenated rosin, and polybutene. U.S. Pat. No. 2,807,555 discloses a thermo-fluid vehicle comprising a mixture of the reaction product of stearic acid and an aliphatic amine, a natural vegetable wax, polyethylene glycol, and optionally a phosphorated tall oil. U.S. Pat. No. 2,842,454 discloses a thermo-fluid vehicle comprising paraffin, natural vegetable wax and aluminum stearate.
The use of known thermo-fluid ceramic inks has not always achieved entirely satisfactory results in multicolor screen printing because of damage to previous prints from the heat of succeeding screen applications. For example, screen markings from one screen application can appear on the print of a preceding screen application. Or alternatively, a succeeding screen application can pick-off or lift-off part of the print of a previous screen application.
Moreover, the increasing use of disposable, nonreturnable containers has introduced a further cost consideration. In addition to the foregoing requirements, an ink used for screen printing, either for single or multicolor printing, must also be relatively inexpensive for application to containers designed to be thrown away after a single use.
It is known to use a thermo-fluid ink containing a low-density polyethylene having a melt index of 200 grams per ten minutes as determined by ASTM D1238, a polyterpene resin, wax, pigment and fillers to decorate a plastic substrate.
U.S. Pat. No. 3,872,044 discloses thermo-fluid ink adapted for application by a screen process to ceramic, metal or plastic ware, the ink comprising a solid thermoplastic polyamide resin formed by reacting a dicarboxylic acid, such as a dimerized fatty acid, with a linear diamine such as hexylmethyldiamine. The solid polyamide has a molecular weight or can be plasticized with a sufficient amount of a compatible plasticizer to have a melting point within the range of about 85.degree. C. to about 120.degree. C. The ink also includes a sufficient amount of a pigment to impart color or opacity, and can include an organo silane.